Thermoset resins, including unsaturated polyesters, are commonly employed in a variety of fabrications, such as casting materials, fiber reinforced materials and coatings. Unsaturated polyester resins may be defined as the condensation products of dicarboxylic acids or anhydrides with difunctional alcohols, typically dissolved in a reactive diluent such as styrene. Since the commercial inception of unsaturated polyester resins, styrene has been a primary reactive diluent due to its low cost, availability, ease of use and resulting excellent mechanical properties. However, in recent years, government regulations have restricted styrene emissions in open molding facilities due to harmful environmental and health effects.
A number of alternatives has been sought to reduce emissions and as substitutes for styrene. Higher molecular weight derivatives of styrene, such as vinyl toluene and t-butyl styrene, have been utilized as reactive diluents and resulted in decreased emissions. However, emissions are still significant in these resins, and such alternative styrene analogs are volatile organic compounds. The addition of paraffins to unsaturated polyester resins has been shown to reduce emissions by forming a thin film on the resin surface. While suitable for some applications, this solution is unacceptable for other applications due to a decreased interlaminate adhesion.
In most cases, low volatile acrylate and methacrylate monomers have been inadequate as replacements for styrene in unsaturated polyester resins since atmospheric oxygen severely retards polymerization, resulting in an under-cured “tacky” surface. Furthermore, the poor copolymerization of most acrylates and methacrylates with maleate and fumarate moieties in the polyester backbone often results in inferior mechanical properties compared to styrenated unsaturated polyester resins.
In this application, the term (meth)acrylate will be understood to include both acrylate and methacrylate compounds.
Previous patents have described non-styrenated unsaturated polyester resin compositions.
U.S. Pat. No. 5,747,597 describes a curable resin comprised of an unsaturated polyester, epoxymethacrylate and/or urethane methacrylate, combined with oligoalkyl ether-monoalkoxy methacrylates. The resin was described in many cases to air dry “dry-to-touch” after the addition of wax, free-radical initiator, catalyst and exposure to elevated temperatures (130-145° F.). Tensile testing of the cast resin revealed high elongation and low strength properties, which is in sharp contrast to data using the same polymers dissolved in styrene that demonstrated considerably higher tensile strengths.
International Patent Publication No. WO 99/23122 describes the use of hydroxyethyl methyl methacrylate, hydroxyethyl propyl methacrylate and/or urethane hydroxyethyl methacrylate with polyesters and vinyl esters. The resulting tensile and flexural properties of the cured products were comparable to styrenated unsaturated polyester resins, but tack-free cure in thin films and thin laminates tends to be a problem with these resins. Additionally, based on our evaluation, the storage stability of resins described in this publication is very poor.
Additional U.S. patents which describe non-styrenated unsaturated polyester resin compositions are U.S. Pat. No. 5,908,875, U.S. Pat. No. 5,739,240, U.S. Pat. No. 6,277,939, U.S. Pat. No. 5,773,531 and U.S. Pat. No. 6,037,385.
An ongoing goal in the unsaturated polyester industry has been to develop improved non-styrenated, free-radical curable resins which have processing similar to current styrenated resins, benefits in mechanical and physical properties and tack-free cure in thin films.